This invention relates to micro-mechanical elements and, more particularly, to an improved design and method of manufacture of such micro-mechanical elements.
There is a growing interest in the fabrication of micro-mechanical elements using fabrication processes developed for semi-conductor electronic components. As fabrication processes have improved, micro-mechanical devices have become smaller which has provided them with the ability to switch more quickly. This has lead to micro-mechanical elements being employed as micro switches, accelerometers, optical elements and capacitance switches amongst other things. If, however, micro-mechanical elements are to be employed as logic elements or memory elements which are able to compete with semi-conductor processors and memory chips their size must be shrunk to around a micron squared in area. There are a number of problems in shrinking micro-mechanical elements to this size. For example, many conventional micro-mechanical elements have a cantilever arm which must be sufficiently rigid to maintain its shape and make contact with electrodes only in the desired positions. Furthermore, in many examples the cantilever arm must have sufficient resiliency to return to a rest position when not influenced by electrostatic or electromagnetic fields. These considerations limit the minimum thickness a conventional cantilever arm can be made. They also limit the speed of switching as electrostatic energy has to be used to overcome the mechanical rigidity of the cantilever.
Conventional micro-mechanical elements have thus had thick cantilever arms which require large electrostatic switching electrodes, which has, in turn, limited the minimum size and maximum switching speed of micro-mechanical elements.
The object of the present invention is to provide a micro-mechanical element having a smaller size and faster switching than conventional elements.
According to the present invention there is provided a micro-mechanical element comprising:
a discrete switching element; and
switching means for applying force to the switching element to move the switching element between two stable positions.
The switching means may apply either electrostatic or electromagnetic force.
The switching element may be suspended in fluid which regulates its movement.
The micro-mechanical element may have one or more protrusions for abutting corresponding indentations on the switching element. The number of protrusions is preferably three.
The micro-mechanical element may be used to implement a memory element. Where a non-volatile memory element is required, adhesion ensures that the switching element remains in the stable positions.
A coating may be applied to surfaces of the micro-mechanical element or switching element to control adhesion between these elements. The coating may preferably be a polymer or oil.
The micro-mechanical element may be used to implement an optical switch. The switching element may then be fabricated from a semi-transparent material and one stable position may facilitate constructive interference between reflected and transmitted components of light, and the second stable position may facilitate destructive interference between the reflected and transmitted components of light. Alternatively, the position of the switching element may be used to modulate transmission of light through a transparent micro-mechanical element. The optical switch may be used as a pixel in a display.
Another implementation of an optical switch according to the invention has either a reflective switching element and an absorbent surface or a reflective surface and an absorbent switching element. Then, in one of the stable positions the light is incident on the switching element and in the other stable position light is incident on the surface, and light is thus either absorbed or reflected.
The micro-mechanical element may further comprise a pivot, and the switching means may then operate to tilt the switching element between two stable positions. A different circuit may be completed at each of the stable positions and the switching means may operate to tilt the switching element in accordance with the requirements of a logic gate.
Alternatively, the micro-mechanical element may further comprise a semi-conductor base with doped regions arranged such that the switching element causes currents to flow between the doped regions when the switching element is in one of the stable positions. Another alternative is to use a capacitance circuit to respond to the position of the switching element.
The abovementioned devices have the advantage that they may be used in integrated circuits in place of conventional semi-conductor devices, providing small, fast-switching and temperature resistant circuit elements.
Also according to the present invention there is provided a method of manufacturing a micro-mechanical element, the method comprising the steps of:
providing a metal electrode on a substrate;
providing a temporary layer over the electrode;
forming a switching element on the temporary layer; and
removing the temporary layer such that the element is discrete and movable by forces applied by the electrode.
Preferably the electrode, temporary layer or element are provided by photolithographic, chemical deposition, sputtering or metal vaporisation techniques.
The method may further comprise the steps of:
providing a protective layer on the temporary layer;
etching the temporary layer where it is not covered by the protective layer to form a space;
forming the switching element by electroplating in the space etched in the temporary layer; and
etching into the substrate to free the electroplated switching element.
Alternatively, the method may further comprise the steps of:
providing a further temporary layer over the switching element;
forming a second metal electrode on the further temporary layer; and
removing the second temporary layer such that the element is discrete and movable by forces applied by the second electrode.
The above methods are advantageous in that they allow fabrication of elements according to the invention using known manufacturing techniques such as photolithography, chemical deposition, sputtering or metal vaporisation.